We have previously prepared and tested a variety of formyl-tripeptides, including the prototype formyl-methionly-leucyl-phenylalanine (CHO-Met-Leu-Phe-OH), for their ability to stimulate various rabbit peritoneal neutrophil functions, including lysosomal enzyme release and chemotaxis. In an attempt to unify the available data into a testable hypothesis, we have recently proposed a working model of the interaction of formyl-tripeptides with the receptor of rabbit neutrophils. Our proposed studies will attempt to define further the molecular pharmacology of the peptide receptor of rabbit neutrophils. Analog studies will be carried out to test the predictions of the proposed model of the ligand/receptor interaction. Receptor binding of agonist will be compared with agonist induced metabolism of membrane phospholipids, as well as lysosomal enzyme secretion. These studies will be extended to receptors isolated from the human promyelocytic HL-60 cell line. Solubilized receptors will be purified by affinity chromatography and HPLC and optimal conditions established for their reconstitution into lipid vesicles. In this way, receptor-ligand interactions can be evaluated in a controlled environment that simulated the cell membrane. The solubilized and reconstituted receptors will be evaluated via specific binding of ligand. The data will be analyzed to obtain equilibrium (i.e., Kd and receptor number) and kinetic (thermodynamic) constants. Receptor selectivity, viz a viz the receptor model, will also be determined. The information provided by this proposed study may help to explain the various ways in which neutrophils modulate receptor mediated functions, including secretion.